METHODS OF EXHIBITING. 



3. "With the same lantern, and the same slider, a picture of any 

 desired magnitude can be produced. To increase the picture, it is 

 only necessary to push in the lens B, so as to bring it closer to the 

 slider, and to remove the screen F to a greater distance. But it 

 must be remembered, that every attempt to enlarge the picture 

 will not only be attended with greater indistinctness, owing to 

 spherical aberration, and more appearance of colours at the 

 edges of the figures, owing to chromatic aberration, but also the 

 brightness of the picture will be greatly diminished, since it is 

 evident that the greater the surface over which the light by 

 which the slider is illuminated is diffused, the more faint, in the 

 same proportion, will the picture on such surface be ; and, since 

 the magnitude of such surface increases in the same proportion 

 as the square of its linear dimensions, it follows that when the 

 picture has double the height or width, it will be four times less 

 bright. (See Tract on " Optical Images.") 



4. The ~body of the lantern should be large, so that it may not 

 become inconveniently heated. The best oil should be burnt in 

 the lamp, so as to diminish the smoke and disagreeable odour. 

 The glass chimney of the lamp should be made as high as possible, 

 and the wick should be of large calibre. 



5. The pictures on the sliders should be as large as possible, in 

 order to ensure sufficient illumination on the screen. With a 

 given magnitude of picture on the screen, and a given force of 

 lamp, the illumination will be proportional to the magnitude of 

 the slider. If a small slider be used to produce a picture on the 

 screen of a given magnitude, the confusion arising from both 

 kinds of aberration will be greater than if a larger one were 

 used. 



6. There are two ways of exhibiting the pictures on a screen : 

 in one, the lantern is placed in front of the screen, with the 

 spectators ; in that case the picture is seen by the light reflected 

 from the screen, after having been projected upon it by the 

 lantern. 



Care should therefore be taken that no light shall penetrate 

 through the screen, since all such light would be lost, and the 

 picture on the screen would be proportionally more faint. A 

 screen composed of muslin, or any other textile fabric, would in 

 such case be defective, inasmuch as more or less of the light would 

 penetrate it. The best sort of screen is one made of strong white 

 paper, pasted on canvas, and stretched on a frame, as canvas is 

 in a picture. 



7. When the magic lantern is used for purposes of amuse- 

 ment, rather than those of instruction, it is generally found 

 desirable to use a semi-transparent screen, the lantern being 



o2 195 



